The parameter used most frequently as a basis for chemotherapy is the minimal inhibitory concentration (MIC). The results of this test are widely influenced by the method applied. A test under in vivo-like conditions is most likely to serve as a basis for rational therapy. The MIC values, measured in different body fluids under varying conditions, also vary, however. Therefore it is unlikely that any measured MIC corresponds with the actual MIC at the site of the infection. The consequence is that a standard test has to be applied which always gives reproducible results. The measured values can thus only to be regarded as relative data in comparison with the concentration of the drug at the site of the infection. Pathogen elimination in vivo during therapy and population-kinetics under the influence of varying concentrations of a drug in vitro are discussed with to the evaluation of the efficacy of different dosages of an antibiotic.